applied fracture mechanics

346Applied Fracture Mechanics(that is, to make the results not conditional on having a crack to begin with) all those resultsare to be multiplied by p*. This parameter is also not well defined, but cancels out in therelative benefit of ISI as expressed by Eqn. (29), because it appears in both the numeratorand denominator.Figure 5. Schematic representation of ak from known subcritical crack growth results and specifiedinspection (Harris DO et al., 1983).Thus, the relative benefit of ISI is independent of both the conditional initial crack sizedistribution and the probability of having a crack. These two factors are currently sources ofconsiderable uncertainty, and their absence from the relative benefit allows the influence ofISI to be assessed with greater confidence then if they were present in Eqn. (29). This resultis for only one dimensional crack. The degree to which it is applicable to the more realisticcase of two-dimensional cracks will be addressed in the following section.4.2. Probability of detection curvesNDE techniques do not detect all cracks, but can be thought of as detecting cracks with aprobability that depends on crack size. Detecting a crack depends on many factors,including the dominant degradation mechanism; the flaw location (e.g. surface or buried),orientation, and shape; material (e.g. stainless and ferritic steel); the inspector’s training andexperience; and the inspection method and procedure. In the literature, the POD has beenused to define the capability of specific NDE techniques and inspection teams (Doctor et al.,1983), (Taylor et al., 1989) and (Heasler, 1996). Usually, however, the POD curve is given asa function of the defect size (depth) with all other factors governing detection assumed to beheld constant for each given POD curve. In this paper, the POD curve is taken to be themeasure of inspection reliability.